Pixelated/Blocky Effect in Smoke Render in Blender 2.82a?
TL;DR,
Use png, exr, jpeg with high quality... image format to save as sequence and encode them in other program to video. eg. Adobe Premiere (Media Encoder), DaVinci Resolve

Depend on your video encoder, keep in mind that most of the video format are lossy-compressed by default, which means some data are actually loss in the video from the source.
And most of the encoding algorithm assume that one frame should be similar to next frame, to reduce the space requisition for similar color. So if your explosion make &quot;fast and high dynamic color change&quot;. Your video will perform a low resolution, or blur, chunk image sequence, if your video bit-rate is set as a relatively low value.
You can set your H.264 render parameter to lossless to suppress that mosaic effect:

Or you can use AVI Raw format to save a BMP-sequence-like video format.